The invention is directed to a drive arrangement for a motor vehicle and, in particular, to a drive arrangement in which the motor vehicle is driven by at least one electric motor which is powered by a generator arrangement driven in turn by an internal combustion engine.
Conventional internal combustion engines of the type customarily used in motor vehicles can generate a desired output of determined magnitude at different engine speeds. In the family of characteristic curves or performance data for torque and engine speed, the operating points of identical output follow approximately hyperbolic characteristic lines corresponding to the relationship by which the output is substantially equal to the product of the engine speed and torque. However, different values of such operating parameters as specific fuel consumption, volume and composition of pollutants in the exhaust or noise development of the internal combustion engine are associated with the operating points of constant output in the performance data. An optimum for one of the operating parameters, e.g. specific fuel consumption, i.e. the fuel consumption with reference to the work performed, could be determined by appropriate selection of the operating points for each desired output.
In conventional motor vehicles driven by an internal combustion engine via a mechanical transmission, the operating points for a desired output can approximate the optimum of the operating parameters only within strict limits, since a selected change in the engine speed can only be made corresponding to the degree of the gear change.
A drive arrangement for a motor vehicle whose wheels are driven by separate electric motors is known from "VDI-Berichte" No. 878, 1991, pages 611 to 622. A generator driven by an internal combustion engine supplies the electric current to the electric motors. An electronic control unit controls the current supplied to the electric motors by the generator depending on the drive output selected by the driver of the motor vehicle via the accelerator pedal. Electric motors and generators suited to drives of this kind are known, for example, from the European Patent Application 0 159 005. A suitable control unit is described in the European Patent Application 0 340 686.
DE 32 43 515 A1 describes a strategy for adjusting the output and speed of the internal combustion engine of an electric diesel locomotive during the starting process. A locomotive is a railborne vehicle in which, unlike motor vehicles for road traffic, relatively constant driving conditions generally prevail within extensive phases of the driving operation. According to this prior art, unnecessarily high speeds of the internal combustion engine which would be the necessary outcome of the low drive output due to the low vehicle speed need only be prevented during the relatively brief phase of the starting process. For this purpose, below a given threshold value of the vehicle speed, the speed of the internal combustion engine is allowed to increase as the vehicle speed increases and in proportion to the rate of increase depending on the reference variable (reference output of the drive), so that when the threshold value of the vehicle speed is reached the speed achieves the reference speed in question and is maintained at this reference speed at higher speeds. Further, each reference speed has a fixed one-to-one correspondence with a reference output. During the starting process, the control unit ensures that the reference value of the electrical output of the generator of the vehicle is changed corresponding to the change in the actual speed of the internal combustion engine. As is clear from the embodiment example, both the increase in speed in the transitional range and the correspondence between the control variable and speed are predetermined arbitrarily as a linear function. There is no mention of the possibility of determining a certain reference output of the internal combustion engine as a function of given optimization criteria (e.g. consumption, noise emission, pollutant emission) with reference to a corresponding family of characteristic curves of the internal combustion engine.
Disturbances in the operation of an internal combustion engine are frequently manifested early on, but not perceived by the driver. Often, a reaction to such disturbances (e.g. shutting off the internal combustion engine) is made only after extensive damage has already occurred.